1. The Field of the Invention
The present invention relates to switched power supplies, and more particularly to high-efficiency switched power supplies that are used to achieve energy conservation and to minimize the generation of undesirable heat in associated electronic circuitry.
2. Background Art
The switching element of a switched power supply is one of the major sources of losses or inefficiency. Losses associated with the switching element are of two types. The first is associated with the switching element itself. These are primarily the result of finite switching time and non-zero conductive resistance. The second type of losses consists of those associated with driving the switching element. A common example is the power loss associated with providing the base drive to operate the switching element.
Bipolar or junction transistors are now considered disadvantageous as switching elements in this context. Such transistors consume excessive amounts of power and require that the base thereof be overdriven by excessive current. The use of a large current to drive a switching element is doubly troublesome in that the charge thereby induced to enter the base of the bipolar switching element must thereafter be drained from the base of the bipolar switching element, if that element is to be turned off. Doing so with large amounts of charge will consume time, reducing switching speed. Also, large base currents are a source of energy loss or undesirable heat generation.
Currently it is preferred as a switching element in a switched power supply to employ a power field effect transistor. Such transistors require virtually no power to remain in either the "on" or "off" state. Power field effect transistors do, however, involve significant input capacitance. This may be envisioned as a capacitance developed between the gate and the source of the field effect transistor. Typically the capacitance associated with a power field effect transistor is in the range of 1000 to 2000 pf. Before such a device can be switched on, that capacitance must be charged and before the device can be turned off, the capacitance must be discharged.
Accordingly, field effect transistors must be driven with a low impedance source if rapid switching is to be effected. Typically this is accomplished using a low value of resistor connected between the gate and emitter of the field effect transistor. This resistor serves to rapidly discharge stored gate voltage. To rapidly charge the gate junction, external current is applied to the gate resistor through a low impedance source. This rapidly charges the gate capacitance.
While the gate junction of a field effect transistor dissipates very little power, the resistor between the gate and emitter normally dissipates a relatively large amount of power. This is particularly the case where low values of resistance are used, as are necessary to achieve rapid switching. Frequently, the resistor between the gate and the emitter is connected in series with a second resistor in order to ensure that voltage applied to the gate does not exceed a predetermined maximum. This additional resistor only further serves to increase the power dissipation associated with the drive circuitry for the field effect transistor.